The quartz crystal microbalance seems to be a very useful tool in electrochemical studies, but, up to now, no attempts have been made to calibrate the microbalance under electrochemical conditions. The aim of this paper is to determine not only the average sensitivity for different active areas but also the differential sensitivity as a function of the radial position of a localized change of mass. In this way, for homogeneous mass perturbation, it is possible to calculate the change of mass from the frequency shift for different active areas by taking into account the average sensitivity value. In the case of localized , the differential sensitivity value allows one to calculate from if the position of the event is known. The calibration procedure was made for an AT‐6 MHz quartz crystal.